1. Field of the Invention
The present invention relates to an input-switching voltage-controlled oscillator and to a PLL-controlled oscillator that uses the input-switching voltage-controlled oscillator, and more particularly to voltage-controlled oscillator that is applied in a PLL-controlled oscillator that is used as a jitter filter incorporated in an optical transponder of an optical transmission system.
2. Description of the Related Arts
A jitter filter is used in an optical transmission system for shaping and recovering jitter, i.e., fluctuation over time in the waveform of an electrical synchronizing signal or electrical clock signal. A jitter filter normally employs a voltage-controlled oscillator implemented by means of a PLL (phase-locked loop) control, i.e., a PLL-controlled oscillator.
FIG. 1 shows the construction of a PLL-controlled oscillator of the prior art that is used as a jitter filter. This PLL-controlled oscillator is generally constituted by phase detector (PD) 1, low-pass filter (LPF) 2, and voltage-controlled oscillator (VCO) 3. Phase detector 1 compares the phases of reference signal fr and feedback frequency signal fo, which is one portion of the output of voltage-controlled oscillator 3, and generates a voltage corresponding to the phase difference of the two signals. This phase difference voltage is supplied to low-pass filter 2. In general cases, the phase difference voltage from phase detector 1 is amplified by an amplifier (not shown in the figure) and then applied to low-pass filter 2. Low-pass filter 2 eliminates the high-frequency components from the phase difference voltage and smoothes the voltage to generate control voltage Vcp.
Since the range of variation in the oscillation frequency need not be particularly great for a PLL-controlled oscillator that is used in a jitter filter, a voltage-controlled crystal oscillator (VCXO) is preferably used as voltage-controlled oscillator 3.
FIG. 2 shows an example of the construction of this type of voltage-controlled oscillator 3. Voltage-controlled oscillator 3 is provided with, for example, quartz-crystal unit 4, oscillation circuit (OSC) 5 that with crystal unit 4 constitutes an oscillation closed loop, and variable-voltage capacitance element 6 such as a variable capacitance diode that is inserted in the oscillation closed-loop. Control voltage Vcp is applied by way of high-frequency blocking resistor 7 to the cathode of variable-voltage capacitance element 6. Oscillation circuit 5 is provided with an oscillation capacitor that is connected to crystal unit 4, and an oscillation amplifier. When the capacitance between the terminals of variable-voltage capacitance element 6 changes due to the application of control voltage Vcp to variable-voltage capacitance element 6, the serial equivalent capacitance of the oscillation closed loop as seen from crystal unit 4 changes, and oscillation frequency fo changes.
In this type of PLL-controlled oscillator, the phases of feedback frequency signal fo, which is a portion of the output of voltage-controlled oscillator 3, and reference signal fr are compared, control voltage Vcp that accords with the phase difference is supplied to voltage-controlled oscillator 3, and output frequency fo therefore imitates the frequency of reference signal fr. Accordingly, the stability of output frequency fo of PLL-controlled oscillator, i.e., the output frequency of voltage-controlled oscillator 3, depends on the frequency stability of reference signal fr, the stability of output frequency fo rising if the output stability of reference signal fr rises.
In an optical transmission system, however, a clock signal is typically transmitted to serve as the reference for the insertion or extraction of information signals. Fluctuations in time are produced in this clock signal during transmission through an optical cable of long or intermediate distance, and the jitter characteristic therefore worsens. The above-described PLL-controlled oscillator is used to restore the jitter characteristic that has deteriorated in this way. In such a case, the clock signal that has been received by way of the optical cable is supplied to the PLL-controlled oscillator as reference signal fr. Jitter, i.e., the fluctuation in time in the waveform, is absorbed by low-pass filter 2, and a clock signal in which the jitter characteristic has been restored can therefore be obtained from voltage-controlled oscillator 3. The jitter characteristic of the clock signal that is outputted from voltage-controlled oscillator 3 is dependent on the jitter characteristic of voltage-controlled oscillator 3 itself. Even in such cases, the frequency stability of the clock signal that is outputted from voltage-controlled oscillator 3 is dependent on the frequency stability of the clock signal that is used as reference signal fr. Thus, if a crystal oscillator having an excellent jitter characteristic is applied in voltage-controlled oscillator 3, the jitter characteristic of the clock signal that is transmitted through the optical cable can be restored while maintaining the frequency stability.
Thus, in a PLL-controlled oscillator of the above-described construction, extinction of the clock signal on the transmission path for whatever reason and resultant failure to receive reference signal fr as input results in the occurrence of abnormalities in output frequency and the inability to obtain the output itself. Thus, in the prior art, detection was performed to determine whether or not reference signal fr was being supplied to the PLL-controlled oscillator that was employed as a jitter filter, and upon detecting that reference signal fr was not being supplied, switching was performed to a synchronizing signal generator such as a crystal oscillator provided separately from the PLL-controlled oscillator and the output of this synchronizing signal generator was employed. However, this necessity to provide a synchronizing signal generator such as a crystal oscillator separate from the PLL-controlled oscillator in such a configuration of the prior art both increased costs and impeded the miniaturization of the jitter filter.
Although output from the synchronizing signal generator could be used at all times regardless of whether the clock signal is received as input or not, thereby obviating the need for a PLL-controlled oscillator to begin with, such a solution lacks compatibility with existing optical transmission systems. Essentially, in an optical transmission system, a clock signal used in a device on the receiving side is typically generated based on the clock signal that is transmitted from a device on the transmitting side, and the use of an independent synchronizing signal generator in the receiving-side device raises the danger of divergence between the clock signals of the transmitting and receiving sides. Thus, in actuality, not only is a PLL-controlled oscillator required, but a synchronizing signal generator is also provided in reserve for such cases as the interruption of the clock signal from the transmitting side.
It is a first object of the present invention to provide an input-switching voltage-controlled oscillator for use in a PLL-controlled oscillator, this voltage-controlled oscillator both being compact and economical and maintaining stable oscillation output in cases such as the interruption of a reference signal.
It is a second object of the present invention to provide a PLL-controlled oscillator for use as, for example, a jitter filter, this PLL-controlled oscillator both being compact and economical and maintaining oscillation output in cases such as the interruption of a reference signal.
The first object of the present invention is achieved by an input-switching voltage-controlled oscillator that includes: an oscillation closed-loop provided with a variable-voltage capacitance element; and a switch for interchangeably selecting one of a plurality of voltages in accordance with a selection signal and applying this voltage as a control voltage to the variable-voltage capacitance element.
The first object of the present invention is also achieved by an input-switching voltage-controlled oscillator that includes: a variable-voltage capacitance element to which a control signal is applied; a capacitor; and a switch for interchangeably selecting one of the variable-voltage capacitance element and the capacitor according to a selection signal and inserting the selected component into an oscillation closed loop.
The second object of the present invention is achieved by a PLL-controlled oscillator that includes: a phase comparator for detecting the phase difference between a reference signal and an output signal; a low-pass filter connected to an output of the phase comparator for outputting a control voltage; an oscillation means for generating the output signal; a selection means for, in accordance with a selection signal, making a transition between a first state in which the oscillation means is caused to oscillate at an output frequency that changes in accordance with said control voltage and a second state in which said oscillation means is caused to oscillate at an output frequency that is unaffected by said PLL-control signal.